1. Field of the Invention
The present invention generally relates to electrical component packaging and particularly to cases for toroidal inductors which are suitable for automatic insertion on circuit boards.
2. Background Description
Circuit board fabrication is often accomplished with the aid of automatic insertion equipment. This equipment automatically handles electrical components and inserts the component leads into holes in the circuit board. In order that electrical components, such as a wire-wound magnetic core, can be inserted on a circuit board with such equipment, the component is often placed on a supporting structure or xe2x80x9ccasexe2x80x9d, that enables automatic handling. A typical existing case design for electrical components includes an open box with holes in the bottom for the wire leads. Once the leads are threaded through the holes, an adhesive or potting compound is used to secure the component to the case. Later the leads are sheared to length.
As technology advances, electrical components become smaller and are packaged more closely. However, it remains necessary to meet safety and operating requirements. As electrical components become smaller and are packaged more closely, the distance between conductors grows smaller and electrical effects which were not significant for larger distances become limiting factors in the design and packaging of electrical components.
Of particular concern are toroidal transformers for mounting on printed circuit boards, because of their relative bulk and higher voltages. An increasingly significant factor affecting the design of case mounted toroidal transformers is xe2x80x9ccreepage distance,xe2x80x9d which is the shortest distance through air along the surface of an insulating material between two conductive parts. Minimum creepage distance requirements increase where air pollution generates high and persistent conductivity caused, for instance, by conductive dust or moisture.
To achieve minimum creepage distance requirements manufacturers have several options under current state of the art. First, they may raise the core to provide the required distance between windings and the terminations. Second, they may terminate the wires outside the case at a point some distance from where the wires exit the case. However, raising the core increases component height, which defeats an advantage of a smaller core. Furthermore, if the wires are terminated outside the case the burden of satisfying minimum creepage distance requirements passes to the circuit board designer who must pay special attention to the layout of the printed circuit board and the location of adjacent components.
What is needed is a way to provide minimum creepage distance without either raising the core or terminating wire leads some distance from where the wire exits the casing. Furthermore, it would be advantageous to provide an approach which is flexible enough to accommodate a wide range of creepage distance requirements.
It is therefore an object of the present invention to provide a transformer casing design which increases the creepage distance.
It is a further object of the invention to provide a transformer casing design which allows the creepage distance to be varied to accommodate different limits in minimum creepage distance, and to do so without changing the size, footprint or pin placements of the case.
Another object of the invention is to have a casing design for minimum creepage distance which allows for smaller core sizes.
It is also an object of the invention to provide a casing design which does not require raising the case in order to achieve minimum creepage distance requirements.
A further object of the invention is to provide a casing design which does not require wire termination some distance from where the wire leaves the case in order to achieve minimum creepage distance requirements.
Yet another object of the invention is a casing design that is inexpensive to manufacture.
The present invention provides a sleeved case design wherein variable creepage distance is provided by sleeves protruding upward into the case and through which wire leads egress from the case at a mounting hole. This does not require raising the core or terminating the wire some distance from the mounting hole. This sleeved case can be built with different sleeve heights to meet different safety distance requirements. The sleeve allows production of a current sense device which is much smaller in physical size than is otherwise acceptable, much less expensive to manufacture, and still meet the creepage and clearance dimensions required by the Safety Agencies.